Mar 2013
Electricity Transmission
Exported Potentials and Profiles Around Earth Electrodes and Opposite-Side Injection for Large-Area Earthing Systems
Mar 2013
National Grid Electricity System Operator
National Grid TO Innovation Team
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Innovation Funding Incentive
Asset Management
The project proposal is divided into five areas of investigation:

  • Prediction of ground surface exported potentials and potential fall-off in the vicinity of earth electrodes. Previous work has demonstrated that exported potentials can be measured fairly accurately using the developed techniques. It is proposed that these tests are carried out at Dinorwig and at Llanrumney test sites

  • Investigation into scalability of low-current injection testing. Previous tests have shown that there is a current dependence of the measured earth impedance in the range 10mA to 5A. In this project, it is proposed to explore and understand these changes over a wider range of current magnitudes including the high current impulse test

  • Investigation of non-linear effects of earth impedance at low-current magnitude and associated polarisation. These laboratory-based investigations will be focussed on clarifying the observed dependence of earth impedance on current magnitude. In particular, it will explore the physical phenomena involved with this behaviour, e.g. i) polarising effects at the electrode-soil interface and the soil-soil particle interface and ii) other non-linear effects including thermal dependence

  • Investigation into frequency effects of earth impedance. An investigation into the frequency effects in earthing system measurements will be undertaken in the laboratory and in the field, to explore further the variability seen from the previous tests and allow a better understanding of the trends

  • Modelling of earth electrodes accounting for non-linear effects. Comparison of the test results, obtained from the practical tests described in the points above, with computer simulations of the electrodes (CDEGS and physical modelling, finite element and boundary element) will allow a better model and equivalent circuits of earth electrodes to be developed accounting for the nonlinear effects.
To determine the safety voltages, the extent of hot-zones and exported potentials accurately is crucial in terms of earthing systems design. This will allow developing efficient and reliable mitigation measures.

In addition, current testing methods/instruments operate in the range of 10mA to 5A. Hence, the scalability of the measurement (to high fault current) is yet to be established. Non-linear effects were seen as a function of frequency and current for low magnitudes. Such phenomena will be investigated and the issue of scalability of test results will be addressed. This forms a significant part of this project.

Furthermore, the credibility and accuracy of predictions using simulation software packages has yet to be fully verified experimentally, and this project willaddress these challenges.